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Hydrocarbons

Tests for alkenes

  • 2 things to know
  • Components 1 and 2

Overview

Observed when bromine water is shaken with an alkane, main organic product formed when bromine water reacts with ethene and why the bromine test distinguishes an alkene from an alkane.

What you need to know

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Key term

Describe the colour change seen when bromine water is added to an alkene.

Key term

What is observed when bromine water is shaken with an alkane?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Describe the colour change seen when bromine water is added to an alkene.

    Model answer

    The orange colour of the bromine water is decolourised almost at once.

  2. 2

    What is observed when bromine water is shaken with an alkane?

    Model answer

    The orange colour remains, because no reaction takes place in the dark.

  3. 3

    Name the main organic product formed when bromine water reacts with ethene.

    Model answer

    The main product is 2-bromoethanol, formed because water as well as bromide competes to attack the intermediate.

  4. 4

    What colour change is seen when bromine dissolved in an organic solvent is added to cyclohexene?

    Model answer

    The orange-brown colour of the solution disappears.

  5. 5

    Explain why the bromine test distinguishes an alkene from an alkane.

    Model answer

    The π electrons of the double bond polarise and attack the bromine molecule so the coloured bromine is used up, while a saturated alkane offers nothing to react with it at room temperature in the dark.

  6. 6

    Describe what is seen when acidified potassium manganate(VII) solution is shaken with an alkene.

    Model answer

    The purple colour of the manganate(VII) fades and the solution becomes colourless.

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